1. Field of the Invention
The present invention relates to an inkjet head that ejects ink toward a recording medium.
2. Discussion of Related Art
Patent Document 1 (JP-A-2004-114362) discloses an inkjet head including a plurality of pressure chambers in communication with a plurality of nozzles and an actuator unit for changing a volume of each of the pressure chambers. In the inkjet head, the actuator unit includes (1) five piezoelectric sheets that are stacked on each other, (2) a plurality of individual electrodes each of which is opposed to a central portion of each of the pressure chambers, and (3) a plurality of common electrodes each of which is provided over the plurality of pressure chambers.
The plurality of individual electrodes are disposed on an upper surface of a first layer of the five piezoelectric sheets and between a second layer and a third layer of the piezoelectric sheets. The common electrodes are disposed between the first and the second piezoelectric sheets and between the third and a fourth piezoelectric sheets. Respective portions of the first, the second, and the third sheets that are interposed between the individual electrodes and the common electrodes constitute active layers (active portions) that contract in a direction perpendicular to a direction of polarization thereof when an electric field is applied thereto. The fourth and a fifth piezoelectric sheets constitute non-active layers. In the actuator unit, when the electric field is applied to the active layers, a difference in strain in the polarization direction is generated between the first to third piezoelectric sheets and the fourth and fifth piezoelectric sheets, so that each of respective opposed areas of the five piezoelectric sheets that are opposed to the pressure chambers is deformed into a convex shape toward the corresponding pressure chamber, so as to constitute a piezoelectric unimorph. Thus, in the actuator unit, so-called “fill before fire” method can be performed. In the “fill before fire” method, the volume of the pressure chamber is once increased so as to introduce ink into the pressure chamber and then the volume of the pressure chamber is decreased so as to apply an intense pressure to the ink accommodated in the pressure chamber.
More specifically, during a waiting time or when ink is not ejected through the nozzles, the common electrodes are kept at a ground potential and a predetermined electric voltage is kept applied to the individual electrodes. In this state, each opposed area is deformed into the convex shape toward the corresponding pressure chamber, so as to decrease the volume of the pressure chamber. When a printing operation is performed in which ink is ejected through the nozzles, the individual electrodes are returned to a zero potential, whereby the piezoelectric sheets return to their initial or normal positions so that the volume of the pressure chamber returns to its normal or initial state, that is, the volume of the pressure chamber is increased from the volume thereof during the waiting time. Thus, a pressure wave is generated in the pressure chamber. Then, when the predetermined electric voltage is again applied to the individual electrodes at a timing when the pressure wave turns positive, the volume of the pressure chamber is decreased, so that an intense pressure is applied to the ink accommodated in the pressure chamber, while being influenced by the pressure wave generated by returning the piezoelectric sheets to their initial positions and the pressure wave generated by decreasing the volume of the pressure chamber. Therefore, two pressures are added to each other, and a considerably small energy suffices to apply a high pressure to the ink, leading to enjoying a high efficiency of the actuator unit.
However, in the inkjet head disclosed in the Patent Document 1, when the printing operation is not performed or during the waiting time, the electric field is kept applied to the active portions interposed between the individual electrodes and the common electrodes. That is, during the waiting time, each opposed area is kept deformed into the convex shape toward the corresponding pressure chamber for a considerably long time compared to a time period when the printing operation is performed. When the electric field is applied to the piezoelectric sheets for a long time, the piezoelectric sheets deteriorate with respect to polarization so that an amount of deformation of the piezoelectric sheets gradually decreases, leading to lowering a pressure applied to the ink accommodated in the pressure chamber.